The question that is crucial here is: How can you deny love in the name of love?! How can you state that love is the reason that love is wrong?!
I have found a very interesting post about the ethical dilemma of monogamy that states my view on the topic in a quite eloquent manner. The following is an excerpt from that post:
For most poly people out there, there's really nothing wrong with monogamy. Both mono and poly are valid relationship models, and everyone chooses the one which best fits them. I really wish I could wholeheartedly agree with this... however, I must admit that monogamy clashes with my ethics. And here's why:
It makes a person feel guilty about having a feeling that I consider to be one of the most important feelings of all. Our ability to connect with other people, to admire other people for their greatness, to be attracted to what we value (spiritually and physically), the ability to love – is probably our most vital characteristic in this life. Monogamy takes a somewhat controversial stance towards it – this property is considered wonderful in a person if he is single, but once he already has one romantic relationship, this very same property is labeled as extremely immoral and wrong. Well, newsflash, everyone: the ability to fall in love does not miraculously disappear when one finds a partner! OK, so maybe for some people it does. Maybe. For most of us, it does not; and polyamory finds this wonderful. In monogamy, however, it is usually considered very wrong to even be attracted to someone else – even if you never act upon these feelings. So, basically, if you're in a monogamous relationship, and there's this other person that you realize is absolutely awesome, then you should feel somewhat guilty. You should reject this realization.
And this is what ticks me off. For me, it's all very simple, really: Love is great. Sex is great. If a doctrine states that love and sex between two consenting adults is immoral, then that doctrine is wrong.
source: What's Wrong with Monogamy